He said that you'd pick your role, and then move to the banning phase. Meaning you pick your champion later.
My point is that many players who are competent playing many roles do not know what role they want to play until they see some other things happen.
For example, maybe I like to mid or jungle most of the time, but if I see an enemy Blitzcrank pick I now want to play support, because I'm really good at shutting down enemy Blitzcranks.
And what's bad about enforcing the meta?
High elo team builder means I can only play fotm champions today. Picking ADC Varus leads to infinite queue times for me. This is obviously bad, yes?
And "Support" players would still troll in normal game types anyway.
This system offers them a new incentive to troll -- queue for support, be a jerk, either get dodged (no penalty for you) or get to play mid with a much faster queue time.
People respond to incentives; previous cases where we change the mechanics of champion select by less have had a much larger effect on troll rates.
You could also just have a "fill" option, so that your third point in the second list is still viable.
If I pick "fill", I'll get matched with "top" "mid" "adc" "jungle" and I won't have the flexibility to pick a role based on what I see get picked, like I currently do in draft mode. Given the expected difference in role popularity, picking "fill" is basically the same as picking "support".